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Summary of leak detection methods for high efficiency filters

1. Principle of Sodium flame method: The principle of Sodium flame method is to spray and dry sodium chloride aqueous solution to form sodium chloride aerosol with a median diameter of about 0.4 m. The dust-containing air is sampled before and after the tested high-efficiency filter material, and the value of photocurrent related to the dust concentration is measured in the sodium flame photometer. Test principle: Test Dust source is monodisperse phase sodium chloride salt spray, "quantity" is the brightness of hydrogen flame when Salt Spray is contained, the main instrument is flame photometer. The salt water splatters in the Compressed air, drying to form a tiny salt spray that enters the air duct. The samples were sampled before and after the filter, and the salt spray sample made the hydrogen flame bluer and brighter. The flame brightness is used to judge the concentration of salt fog in the air and to determine the filtering efficiency of the filter. The average diameter of salt fog is 0.4 m according to the National Standard, but the measured result is 0.5 m. The European measurement of the mean diameter of salt spray particles was 0.65 m. The Sodium flame method is no longer used in Europe with the popularization of other detection methods. The domestic authorities concerned are revising the original national standards, whether to abolish or continue to use the sodium flame method, the opinions have not yet been implemented.

2. According to the "code for construction and acceptance of clean rooms" (JGJ71-90) , the high efficiency filter must have tested the air volume, and designed to run between 80%-120% of the air speed, for the concentration of particles on the windward side of the tested HEPA filter, it must be 3.5104 pc/l for the particle size 0.5 m, and 3.5106-3.5107 pc/l for the particle size 0.1 m. Using the particle counter scanning method with the minimum sampling volume > 1 L/min, the high efficiency filter installation joint and the main section are scanned. The detection point should be 20-30 mm from the surface to be measured, and the probe should move at the speed of 5-20 mm/s, scan the whole section, the sealing head and the mounting frame of the inspected filter. In the "clean room construction and Acceptance Code" , by the high-efficiency filter downwind leakage concentration converted into the penetration rate to measure whether qualified. Actual problems: HEPA filters are generally carried out after the system air volume and the air volume of each outlet are adjusted and balanced. According to the specifications, the deviation between the air volume of each outlet and the designed air volume is less than 15% , this satisfies the condition that the inspected air outlet is carried out at close to the design wind speed. So when the air balance is good to timely high-efficiency filter leakage detection work. In Engineering, the minimum particle size channel of a particle counter is 0.3 m for a purification system of more than 100 grades. Therefore, the particle concentration of the tested hepa filter on the upwind side should be 0.5 m, and the particle concentration should be 3.5104 pc/l, while the concentration of atmospheric dust should be 5.3104-2.5105 pc/l, which reaches the upper limit of the particle counter. In many purification systems, the fresh air entering the circulating air conditioning box is treated by a filter with a concentration much lower than the concentration of atmospheric dust, and the concentration of the mixed return air will be lower. Therefore, the introduction of atmosphere into the installed clean air-conditioning system is a problem worthy of careful consideration. In order to ensure the requirement of particle concentration in the upstream of the air filter, it is an ideal method to introduce the artificial aerosol with uniform concentration in the upstream without disturbing the balance of air flow. How much aerosol concentration is required depends on the efficiency of the HEPA filter and the resolution of the particle counter. Particle counter minimum bit is a bit, the minimum number is zero. Test instruments generally have a dead zone, if the required concentration of less than 10 downstream is qualified, these data is valid, in accordance with the principles of statistics is difficult to ensure. At present, the filtration efficiency of 0.3 m particles is classified as HEPA FILTER BY DOP test in USA. Therefore, when using the counting scanning method to detect leakage, the particle size measured should be 0.3 m, which requires the upstream concentration better. According to the efficiency of HEPA filter 99.97% , the concentration of particles with diameter 0.3 m in upstream air should be at least 6104 pc/l. If a high efficiency filter with efficiency of 99.99% is used, the particle concentration in the upstream 0.3 m is about 2105 pc/l, and the particle concentration in the upstream 0.5 m is far more than 3.5104 pc/l. Therefore, the use of the atmosphere as a dust source can not meet the test requirements. High Flow Particle counter is more suitable to detect the installation of high efficiency filter leakage, the larger the air sample, the more representative the test results, the higher the accuracy. The sample size required by the specification is 1 L/min, while the sample size used by us is 0.1 CFM (2.83 l/min) to meet the requirements of the testing instrument, but this will affect the efficiency of leak detection. Taking the 610610 high efficiency filter as an example, the sampling speed is 20mm/s when the moving sampling head is used under the high efficiency filter, the rectangular 10011.33 sampler takes at least 244 seconds, and the sampling speed of the circular sampler is slower, it takes longer to scan for leaks. However, the calculation method of scanning rate and acceptable observation counting method in ISO14644-3 metrology and test methods is difficult for engineers and technicians to master, suitable sampling rate and acceptable observation counts are recommended for different levels of high efficiency filters to facilitate the practical operation of engineering testers.

 

 3. DOP positive pressure leak detection staff must pay attention to the specification of the sample time, upstream particle concentration and other parameters in practical application of Operability. When participating in the leak detection of high efficiency filter in a certain pharmaceutical factory, a large number of stable micro-particles are produced by laskin DOP generator in combination with the JIS-b-9927 standard of Japan and Fedstd-209d standard of America, so that the upstream particle concentration 0.3 m is greater than or equal to 1105 PC/L. DOP particles are uniformly injected into the system through the Compressed air upstream of the filter (cold generation) . As the concentration is generally greater than 1105 PC/L and out of the measuring range of the detector, a dilution device is connected in series before the detector, the measured concentration is within the measuring range of the measuring instrument. For high efficiency filter with 99.97% ex-factory efficiency, the measured transmissivity should be less than 0.06% , that is, the measured particle concentration should be less than 0.06% C1(C1 is the upstream concentration) . When the concentration of particles emitted by the particle generator upstream of the filter is more than 0.3 m, it is 3.3106 PC/2.83 l, if a design air volume is 15000m3/h, the downstream limit concentration of purification air conditioning system is 1960pc/2.83L. After the Aerosol concentration in the particle generator is stable, the leakage test of the high efficiency filter is carried out with the particle counter. The particle counter must simultaneously scan the outer frame and the inner surface of the HEPA filter twice, and the upper reaches of the filter must also sample and test the aerosol concentration in the air duct twice, and the sampling period and the sweeping surface must be equal, start and end at the same time as possible. Taking the HEPA filter 610610 as an example, it is suggested that the sampling period should be one minute and the rate should be 50mm/s. Prior to leak detection, a 500mm high enclosure is placed around the HEPA filter to prevent the airflow outside the HEPA filter frame from forming eddy currents that affect the results of the scanning test. The main reason for the leakage of the filter is that the sealing effect of the outer sealing ring is not good, or the filter material is not smooth and tight enough. In practical application, the installation leakage mainly comes from the outer frame, the quality of the outer frame sealing strip is closely related to the leakage. As a result, leakage detection in the project will be in the HEPA filter to add around the enclosure, reduce the lower outlet area of the vent enclosure, the enclosure formed a certain positive pressure, if the HEPA filter installation leakage, particles will leak out through the enclosure. At this time, the particle counter can be 50 mm/s speed in the high-efficiency filter at 25 mm scanning 1 minute, can determine whether the filter leakage, this is the "positive pressure leak detection method. ". For the cleaning workshop with a large number of high-efficiency filter Tuyere, the test effect of high-efficiency positive pressure leak detection method is equivalent to the traditional inner and outer frame scanning method, but the efficiency is doubled. Taking a purification air conditioning system with 60 air vents of 610610 mm hepa filters as an example, it usually takes at least two hours to scan with the traditional method, but only one hour with the positive pressure leak detection method, which saves more than half of the time. 4. Principle of oil fog method: the source of dust is oil fog, "quantity" is turbidity of air containing oil fog, the instrument is turbidimeter. According to the turbidity difference of the gas sample, the filtration efficiency of the filter to the oil mist particles is determined. The oil mist diameter of paraffin oil is 0.3-0.5 M. The average diameter of oil mist prescribed by Chinese standard is 0.28-0.34 M. There is no specific regulation on the type of oil. Although Chinese standard stipulates that oil mist method can be used, domestic manufacturers prefer to use another sodium flame method with the same standard specification, only some manufacturers still use the oil mist method when measuring the filter material. 5. Particle counter method test principle: the dust-containing gas flow through the strong light illumination area at a very small speed, the dust particles in the measured air in turn through, each dust particle will produce a light scattering, forming a light pulse signal, according to the magnitude of the light pulse signal is proportional to the size of the particle surface, the particle number and brightness are measured by the Photomultiplier tubes, and the filtering efficiency is determined. The purpose and principle of Pao method: to detect the leakage of hepa filter after field installation is to check whether the HEPA filter and its static pressure box have leakage or whether the leakage is within the scope allowed by the standard. If the high-efficiency filter device after leak detection is qualified, can ensure the safe and reliable operation of the clean room, at this time indoor cleanliness is still not up to the standard, should be from other aspects of the clean room to find the cause. At the tested hepa filter, the Pao Aerosol was sent to the wind side as the dust source, and was sampled by photometer at the downwind side. The scattered light generated by the dust-containing gas when it passed the photometer was converted into electricity by photoelectric effect and linear amplification, the collected air sample is quickly displayed by micro-ammeter through the diffuser chamber of the photometer. The light intensity is different because of the diffusion of particles

 

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